Many times it is desirable to focus a plurality of light sources into a spot, so that a bright source image is formed. For lasers, such applications arise in laser fusion, laser surgery and cutting applications, and in communications.
For conventional lasers with beams of circular symmetry, standard optical lenses may be used to form bright spots. A problem arises, however, in attempting to focus a plurality of diode lasers onto a small spot or zone, i.e. as small as 20 microns on a side of a square entrance aperture, but possibly larger depending on the system specifications. In a diode laser or diode laser bar, light emerges from an active layer or region at a cleaved facet or edge of the diode in a slit-like pattern which is oblong in character. Typical dimensions for a slit in a laser bar might be one micron high by 400 microns wide, with light emerging at a large divergent angle from the small dimension and a significantly lesser divergent angle from the large dimension. Construction of a double-heterostructure, phased-array diode laser with an emitting geometry similar to the one mentioned is disclosed in an article entitled "Phased Array Diode Lasers" by W. Streifer in Laser Focus. June, 1984. In this application the term "diode laser" will be used interchangeably with "laser bar" or "laser array" .
The small size of the diode laser and the wide divergent beam makes the problem of focusing a plurality of beams unlike the same problem associated with conventional lasers. Moreover, to focus an image of the narrow source dimension on a spot with an aperture of 200 microns in diameter, for example, magnification would be required for the narrow slit dimension mentioned above, while reduction would be required for the large slit dimension.
In Sprague et al. U.S. Pat. No. 4,428,647 disclose a plurality of diode lasers imaged to a plurality of spots through a pair of lenses. A first lens system is a lenticular array which changes the angle of divergence from a wide angle to a narrow angle for each diode laser. In this system, a lens element of the lenticular array is associated with each diode laser. A second lens acts as an objective, focusing light to an image plane where a plurality of spots are formed, one corresponding to each diode. Note that total convergence of the beams is not achieved.
An object of the invention is to devise an optical arrangement for focusing a plurality of diode lasers into a single bright focal region.
A further object is to devise a laser arrangement for concentrating a high amount of energy in a small zone.